Power/Performance Bits: Dec. 15

Columbia Engineering researchers powered an integrated circuit from adenosine triphosphate (ATP), the energy currency of life. They achieved this by integrating a conventional solid-state CMOS integrated circuit with an artificial lipid bilayer membrane containing ATP-powered ion pumps, opening the door to creating entirely new artificial systems that contain both biological and solid-state components.

Ken Shepard, professor of electrical engineering and biomedical engineering at Columbia, notes that despite its overwhelming success, CMOS solid-state electronics are incapable of replicating certain functions natural to living systems, such as the senses of taste and smell and the use of biochemical energy sources. Living systems achieve this functionality with their own version of electronics based on lipid membranes and ion channels and pumps, which act as a kind of ‘biological transistor.’ They use charge in the form of ions to carry energy and information — ion channels control the flow of ions across cell membranes.

In living systems, energy is stored in potentials across lipid membranes, in this case created through the action of ion pumps. ATP is used to transport energy from where it is generated to where it is consumed in the cell. To build a prototype of their hybrid system, the team packaged a CMOS IC with an ATP-harvesting ‘biocell.’ In the presence of ATP, the system pumped ions across the membrane, producing an electrical potential harvested by the IC.

“We made a macroscale version of this system, at the scale of several millimeters, to see if it worked,” Shepard said. “Our results provide new insight into a generalized circuit model, enabling us to determine the conditions to maximize the efficiency of harnessing chemical energy through the action of these ion pumps. We will now be looking at how to scale the system down.”

Non-reflective nanopillars

Engineers at the University of Illinois at Urbana Champaign and the University of Massachusetts at Lowell developed a new anti-reflection coating for devices like LEDs, solar cells and sensors. The coating is a specially engraved, nanostructured thin film that allows more light through than a flat surface, yet also provides electrical access to the underlying material – a crucial combination for optoelectronics.

At the interface between two materials, such as a semiconductor and air, some light is always reflected. This limits the efficiency of optoelectronic devices. If light is emitted in a semiconductor, some fraction of this light will never escape the semiconductor material. Alternatively, for a sensor or solar cell, some fraction of light will never make it to the detector to be collected and turned into an electrical signal. A model called Fresnel’s equations describes the reflection and transmission at the interface between two materials.

“It has been long known that structuring the surface of a material can increase light transmission,” said study co-author Viktor Podolskiy, a professor at the University of Massachusetts at Lowell. “Among such structures, one of the more interesting is similar to structures found in nature, and is referred to as a ‘moth-eye’ pattern: tiny nanopillars which can ‘beat’ the Fresnel equations at certain wavelengths and angles.”

The researchers used a method of metal-assisted chemical etching, MacEtch, to engrave a patterned metal film into a semiconductor to create an array of tiny nanopillars rising above the metal film. The combination of these “moth-eye” nanopillars and the metal film created a partially coated material that outperformed the untreated semiconductor.

“The nanopillars enhance the optical transmission while the metal film offers electrical contact. Remarkably, we can improve our optical transmission and electrical access simultaneously,” said Runyu Liu, a graduate researcher at Illinois.

The researchers demonstrated that their technique, which results in metal covering roughly half of the surface, can transmit about 90 percent of light to or from the surface. For comparison, the bare, unpatterned surface with no metal can only transmit 70 percent of the light and has no electrical contact.